In 1973 we activated an epidemiologic study of case-control design to explore relationships of reeproductive capacity and altered risk of breast and ovary cancers. This was an attempt to explain the findings of recent work by MacMahon, Joly, and their co-workers, which showed that women with delayed first childbirth have an increased risk of developing these cancers. Our present observations extend from their original purpose and are directed toward distinguishing between two competing hypotheses that would explain the increased cancer risk resulting from late first childbirth, namely lower fertility (inadvertent delay) as opposed to planned late parenthood (voluntary delay). Another major aim of study is to assess possible relationships of female steroid hormones (e.g., oral contraceptives and postmenopausal replacement drugs) to risk of developing breast and ovary cancers.